A little gas tank trivia: Stock cars had to drag race with full tanks, and tanks were checked with a stick.

Pontiac was known for its cheating at hp rating and other things. The installation of the tank was yet another way that Pontiac cheated.

Instead of mounting the tank so that it would hold 18 gallons, Pontiac mounted the tank tilted upward so that a large air pocket remained, thus, the reason why you can only squeeze 15 gallons into the 18 gallon tank.

This is how I'd tackel the problem. First, I'd verify that the gas indeede comes out of the sending unit. Use an electic feul pump or safely run a test line.

Next, I'd attach a feul source to the line and see what happens. It's has to be one or the other because I realize that you're intellegent enough to know that if you had a vent problem, it would start, and run for however long; then, you'd get a vacume release when you removed the cap.